Multi-mode interference waveguide based switch

ABSTRACT

The present invention relates to device for space selective switching of an optical signal from an input access waveguide to a first selected output access waveguide. Said device comprising a multi-mode interference (MMI) waveguide having at a first side a number, N, of accesses for connection of access waveguides. Said MMI waveguide having a length, in light propagating direction, so that an image at the i:th, i≦N access waveguide propagating into said MMI waveguide will produce N self-images at a second side opposite to said first side, where N is an integer than 1. Said device further comprises reflective means located in said MMI waveguide close to said second side, arranged to reflect said N self-images towards said first side of said MMI waveguide, and means, arranged at said second side, for adjusting the phase of each of said self-images to create a single self-image at said selected output access waveguide.

TECHNICAL FIELD

The present invention relates to a multi-mode interference waveguidebased space switch and more in particular to a compact such multi-modeinterference waveguide switch and a method for switching optical lightsignals.

BACKGROUND OF THE INVENTION

There is a strong need to increase the capacity and flexibility ofphotonic transport networks. The evolving telecommunication networks areincreasingly focusing on flexibility and reconfigurability, whichrequires enhanced functionality of photonic integrated circuits (PICs)for optical communications, as well as compact devices. The interest formulti-mode interference (MMI) effects in integrated optics forsingle-mode transmission systems has recently been increasing. Opticaldevices based on MMI effects have large optical bandwidth, arepolarization insensitive and sustain high fabrication tolerances tomention a few advantages. The operation of MMI waveguide devices isbased on the self-imaging principle and is further described in “OpticalMulti-Mode Interference Devices Based on Self-Imaging: Principles andApplications” by L. B Soldano and E. C. M. Pennings published in J. ofLightwave Technology, Vol. 13, No. 4 Apr. 1995.

MMI waveguide devices have applications in a number of different areas.For instance, as a wavelength selective switch, as described in “Bragggrating assisted MMIMI coupler for wavelength selective switching” bythe present inventor published in Electronics Letters 10^(th) Dec. 1998,Vol. 34, No. 25. In this paper the present inventor describe a noveloptical device for wavelength selective switching. The device is basedon a Bragg grating assisted MMIMI (multi Mode Interference MichelsonInterferometer) coupler.

Another application for a MMI waveguide device is as coupler, asdescribed in the paper “Multimode Interference Couplers with TuneableSplitting Ratios” by J. Leuthold and C. H. Joyner, published in Proc.ECOC 2000, September, Münich Vol. 3. In this paper the authors present anovel, compact multi-mode interference coupler with tuneable powersplitting ratios. The coupler has large tuning ranges and is used tooptimise on-off ratios in interferometric devices or even as a switch.

The need to be able to space switch signals in optical telecommunicationnetworks is apparent. Simple space switching of broadband signals allowsrouting based on for instance available capacity or rerouting aroundparts of the network currently unavailable.

Prior art optical switches currently face problems with high losses,high cross talk or high requirements on fabrication tolerances. They mayalso have stability problems or have high power consumption.

SUMMARY OF THE INVENTION

The object of the present invention is to avoid or reduce theabove-mentioned problems, as well as to provide a compact switchingdevice and a method for switching optical signals.

The present invention thus provides a compact multi-mode interferenceswitch wherein an optical input signal, at a first side of an MMIwaveguide, is selectively routed to an output access waveguide selectedfrom a number of output access waveguides by tuning the phase front ofself-images of said optical input signals, and reflecting saidself-images from a second side of said MMI waveguide, towards said firstside by reflective means.

One advantage with the present invention is that a very compactswitching device is achieved. A further advantage is the provision of astable device having low losses. Yet another advantage is that theswitch thus provided will have low cross talk.

In greater detail, said phase tuning is achieved by providing M phaseshifters, where M is an integer, and where each of said phase shiftersis arranged to individually tune the phase of N self-images at saidsecond side. The MMI waveguide is arranged so that an input light signalat an access waveguide, located at one side in said MMI waveguide, isdivided into N self-images at a second side of said MMI waveguide. Eachof these self-images will have an individual phase at the position ofsaid second side, thus creating a phase distribution over the M phaseshifters. The phase shifters are controllable so as to tune the phasedistribution of said self-images, to coincide with a phase distributionof self-images at said second side that would be produced by an opticallight signal at said selected output access waveguide.

In a preferred embodiment N and M are equal. That is, M=N phase shifterstune the phases of N=M self-images.

According to one aspect of the invention each of said phase shifterscomprises a light transparent part having a first refractive index andmeans for tuning said refractive index.

Said tuning can be performed, according to a preferred embodiment, bymaking the refractive index of said light transparent part sensitive toheat and controlling the heat of said tuning means to effectivelycontrol the refractive index of said light transparent part. Thereby, itis possible to control the optical path length of the incident light andthus the phase for each self-image.

According to another preferred embodiment of the invention, therefractive index of said light transparent part is sensitive to currentthrough or voltage across said part. Controlling the current or voltageand thus the refractive index, then performs the tuning.

According to another aspect of the present invention each of said phaseshifters comprise reflective means. Each of said phase shifters arearranged so that the individual positions of said reflective means arecontrollable in a direction parallel to the direction of propagatinglight in said MMI waveguide. Thereby, it is possible to control thelength of the geometrical distance, and thus the optical path length, ofincident light and thus the phase of said self-images.

According to another preferred embodiment said phase shifters comprise athermo-expansion section which, depending on an applied temperature,moves reflective means in a direction parallel to the direction ofpropagating light in said MMI waveguide.

According to yet another preferred embodiment said phase shifterscomprise micro-mechanical phase tuning means arranged to controllablymove reflective means in a direction parallel to the direction ofpropagating light in said MMI waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a multi-mode interference deviceaccording to a first preferred embodiment of the invention.

FIG. 2 shows a power distribution at the interface B for the wavelengthin question in FIG. 1.

FIGS. 3 a–3 b show different phase distributions.

FIG. 4 shows a schematic view of a device according to a secondpreferred embodiment of the invention.

FIG. 5 shows a schematic view of a device according to a third preferredembodiment of the invention.

FIG. 6 shows a schematic view of a device according to a fourthpreferred embodiment of the invention.

FIG. 7 shows a schematic view of a 1×4 switch according to an embodimentof the invention.

PREFERRED EMBODIMENTS

FIG. 1 shows a schematic drawing of a multi-mode interface deviceaccording to a first preferred embodiment of the invention. To the left,at the interface A, five access waveguides are denoted 101 to 105respectively. The length and width of the waveguide 106 are adapted sothat an input image at an access waveguide will produce 5 self-images atthe interface B. The light propagation direction is denoted 107 and theperpendicular direction 108. It shall be noted that the light also canpropagate in the opposite direction to direction 107.

FIG. 2 shows the power distribution at the interface B for a signalentering the MMI waveguide in FIG. 1 at access waveguide 101. Each powerpeak, denoted 201–205, respectively, represents a self-image and appearsevenly distributed at the interface B. In other words, the X-axis inFIG. 2 is oriented in FIG. 1 in the perpendicular direction 108. Inputfrom the other access waveguides 102–105 will produce similar powerdistributions or self-images at the interface B. The power distributiondifference at interface B between different input access waveguides willbe negligible with a correct design of the MMI waveguide.

The optical bandwidth of the MMI waveguide is inversely proportional tothe number of input and output waveguides. The bandwidth properties ofMMI waveguides are more thoroughly worked out in “Optical Bandwidth andFabrication Tolerances of Multimode Interference Couplers” by P. A.Besse, M Bachmann, H. Melchior, L. B. Soldano, and M. K. Smit publishedin J. of Lightwave Technology, Vol. 12, No. 6, June 1994.

FIG. 3 a shows phase distributions of self-images at interface B in FIG.1 from the top three input waveguides 101–103. That is, a light imageentering the MMI waveguide 106 in FIG. 1 at access waveguide 101 willhave a power distribution according to FIG. 2 and a phase distributionaccording to dashed line 301 in FIG. 3 a. A light image entering the MMIwaveguide 106 at access waveguide 102 will have a power distributionsimilar to the power distribution in FIG. 2 but quite another phasedistribution according to the dash-dot line denoted 302 in FIG. 3 a.Similarly an image entering the MMI waveguide at access 103 will have aphase distribution according to the dash-dot-dot line 303 in FIG. 3 a.

For ease of reference, the phase distribution of an image entering theMMI waveguide 106 at access waveguide 104 is plotted with a dashed linein FIG. 3 b and denoted 304. Finally, the phase distribution of an imageentering the MMI waveguide at the access waveguide 105 is plotted with adash-dot line in FIG. 3 b and denoted 305.

The MMI waveguide 106 in FIG. 1 is reciprocal. At interface B five phaseshifters are arranged and denoted 109–113. Reflector means (not shown)are arranged to reflect incident light. The phase shifters 109–113 alsocomprises means for controllably shift the phase of incident light.Since an input image, entering the MMI waveguide 106 at access waveguide103, will produce a power intensity distribution according to FIG. 2 anda phase distribution according to line 303 in FIG. 3, and since the MMIwaveguide is reciprocal, a power distribution according to FIG. 2 atinterface B, with a phase distribution according to line 303 in FIG. 3,propagating in the opposite direction to direction 107 will produce asingle self-image at access waveguide 103.

To switch a signal, entering access waveguide 102, to exit accesswaveguide 103, the device in FIG. 1, thus operates as follows. Theoptical signal will enter the MMI waveguide 106 and produce fiveself-images at interface B according to FIG. 2, with a phasedistribution according to line 302 in FIG. 3. The phase shifters 109–113are controllably set to shift the phase of the incident light tocoincide with the phase distribution according to line 303 in FIG. 3after reflection and completely passing the phase shifters.

It is of course possible to fine-tune the phase shifters to reproduceaccurate shifting to mimic the phase distribution 303 to the bestpossible extent. This way a reflection is created, at interface B, witha phase distribution according to 303 in FIG. 3, propagating opposite todirection 107, giving a single self-image at access waveguide 103. Thus,a dynamic switching is achieved from access 102 to access 103. Byshifting the phase of the incident light of the signal going into theMMI waveguide the signal can be switched to any of the access waveguides101–105, i.e. including the input access waveguide. There is inprincipal no limitation to the number of input and output accesswaveguides, i.e. a P×Q MMI waveguide switch. If separate input andoutput access waveguides are required then the number of accesswaveguides N is equal to P+Q, alternatively if same input and outputaccess waveguides are used an N×N switch is required.

FIG. 4 shows a device according to a second preferred embodiment of theinvention. This device is a 1×8 switch with one input access waveguide401, and 8 output access waveguides commonly denoted 402. Nineindividually controllable phase shifters 404 are used to control thephase front of an incident light image. Each phase shifter comprises atransparent media having refractive index, which is controllable byapplying a voltage across said media. It is also possible to use athermo-optical material, in which varying the temperature controls therefractive index. By varying the refractive index of the phase shifters404 the optical path length is controllable and thus the phase front ofan incident light image. A broad band reflection section 405 reflectsthe phase-adjusted image to the selected output access waveguide 402.The MMI waveguide 403 comprises an adiabatic taper section 407 with anangle θ_(T) 408. Since the phase if the incident light is tuned over arelatively long distance, i.e. during the travel through the phaseshifters 404, the intensity distribution of the light change during thedistance through the phase shifters 404. By having an adiabatic taperingof the waveguide, i.e. with no coupling to higher modes, a slower changeof the intensity distribution of the incident light will occur. Thus amore effective switching is achieved with substantially reducedcross-talk and also reduced power loss.

In design of the device concern should be taken in that not only theincident light will pass the phase shifters, and the transparent partwith adjusted refractive index, but also reflected light. This needs tobe considered in the design of the phase shifters so that a proper phasetuning is achieved. The phase shifters are properly isolated betweeneach other 406 so that minimal cross talk occurs.

FIG. 5 shows a 1×8 switch according to a third preferred embodiment ofthe invention. Like details are denoted by same numerals. Phaseshifters, commonly denoted 501, comprise a first part 502 having a broadband reflection side 503 arranged to reflect incident light. Said firstpart is arranged to be movable in the direction of light propagation inthe MMI waveguide. Preferably, said first part 502 is spring-loaded totake a first position and is movable in a direction towards said accesswaveguides 401 and 402 to a second position, by a second part 504 ofsaid phase shifter 501. Said second part is a micro-mechanical devicewhich controllable pushes said first part 502 towards said accesswaveguides, thereby shortening the geometrical distance for incidentlight, which enables fine-tuning of the phase of said incident light.Each of said micro-mechanical devices 504 is individually controllableby a control means 505. In this preferred embodiment the MMI waveguidehas no tapered section since the change of phase of the incident lighttakes place over a relatively short distance. Thus no considerationsneed to be taken with regards to changes of intensity distribution.

Refractive index matching means 506 is used to match the refractiveindex in the MMI waveguide 507. This is necessary since the phaseshifters will move and gaps, with for instance air or vacuum, couldoccur in the interface between the phase shifters 501 and the MMIwaveguide. The index matching means is for instance a fluid with samerefractive index as the MMI waveguide. In this case it is preferable toput the fluid under pressure to prohibit formation of bubbles. It isalso possible to use a soft material, which will follow the movements ofthe phase shifters, such as silica-rubber.

FIG. 6 shows a 1×8 switch according to a fourth embodiment of theinvention. Like details are denoted with same numerals. Nine phaseshifters 601 are individually controllable. A broad band reflectionsection 602 is located on a thermo-expansion part 603. Saidthermo-expansion part 603 moves said broad band reflection section indirection parallel to light propagation in said MMI waveguide. Thethermo-expansion part 603 is controlled by a heating element 604. Thus,it is possible to individually control each phase shifter, byapplication of heat through said heating element, to tune the phase ofan incident light image. To avoid thermal cross-talk and to avoidchanging the refractive index of the index matching means 506 bychanging its temperature, a thermal insulation layer 605 is appliedbetween the thermal expansion part 603 and the index matching means 506.

FIG. 7 shows a 1×4 switch with an input waveguide access port 701coupled to an MMI waveguide switch device 702. Four output waveguideaccess ports is denoted 703, 704, 705 and 706, respectively. Anisolation plate 707 is arranged to prevent cross talk between the inputaccess waveguide 701 and the output access waveguides 703–706. Theswitch is approximately 5 mm wide, distance A, and 10 mm high, distanceB with index contrast Δ=1.5%

$\left( {\Delta = \frac{n_{k} - n_{clod}}{n_{clod}}} \right).$The figure is not to scale, for instance is the distance C across theoutput ports 703–706 approximately 1 mm.

1. A device for space selective switching of an optical signal from aninput access waveguide to a first selected output access waveguide, saiddevice comprising: a multi-mode interference (MMI) waveguide having at afirst side a number, N, of accesses for connection of access waveguides,said MMI waveguide having a length, in light propagating direction, sothat an image at an I:th access waveguide will produce N self-images ata second side opposite to said first side, where N is an integer greaterthan 1, reflective means located in said MMI waveguide close to saidsecond side, arranged to reflect said N self-images towards said firstside of said MMI waveguide, and means comprising N individual phaseshifters arranged in line perpendicular to the propagation directIon ofincident light at said second side, for adjusting the phase of each ofsaid self-images to create a single self-image at said selected outputaccess waveguide, wherein said reflective means is arranged on asurface, facing the light propagating direction, of each of said phaseshifters, and wherein each of said phase shifters comprises means foradjusting its position in a direction parallel to the light propagationdirection in said MMI waveguide.
 2. The device according to claim 1,wherein the N self-images, originating from said optical signal enteringsaid MMI waveguide at the i:th access waveguide, has each a phaseP_(n,i) construing a set P_(i) describing a phase distribution of theself-images, at said second side, and said means for adjusting the phaseof each, self-image is arranged to adjust the phase distribution P_(i),at said second side, for self-images from an input access waveguide i,to coincide with the phase distribution P_(j) for a selected outputaccess waveguide j.
 3. Device according to claim 1, wherein each of saidphase shifters comprises means for adjusting the refractive index of atleast a part of said phase shifters.
 4. Device according to claim 3,wherein the refractive index is adjusted by one of adjusting thetemperature and applying a voltage across at least a part of said phaseshifter.
 5. Device according to claim 1, wherein at least a part of saidphase shifter is construed of a thermal-expansive material, saidposition adjusting means is a temperature adjusting means, and that saidtemperature adjusting means is coupled to said at least part of saidphase shifter so that a change in temperature of said temperatureadjusting means change the length of said phase shifter.
 6. Deviceaccording to claim 1, wherein at least a part of each of said phaseshifters are movable in a direction parallel to the light propagationdirection in said MMI waveguide, and wherein said position adjustingmeans is a micro-mechanical device arranged to move said at least a partof said phase shifters in said light propagation direction.
 7. Deviceaccording to claim 1, wherein the refractive index matching means isapplied between said MMI waveguide and each of said phase shifters. 8.Device according to claim 1, wherein each of said phase shifters arearranged side-by-side with an isolation distance in between.
 9. A methodfor switching an optical signal from a first input access waveguide to afirst selected output access waveguide, said first input accesswaveguide and a set of output access waveguides are connected to a firstside of an MMI waveguide, the method comprising: producing a first setof N self-images, where N is an integer, greater than 1 at a second sideof said MMI waveguide from an image appearing at said input accesswaveguide, adjusting phase of each of said N soft-images to create asingle self-image at said selected output access waveguide utilizing Nindividual phase shifters arranged in line perpendicular to thepropagation of incident light at said second side, and reflecting said Nself-images towards said first side of said MMI waveguide by arrangingreflective means facing the light propagating direction of each of saidphase shifters, wherein each of said phase shifters comprises means foradiusting position in a direction parallel to the light propagationdirection in said MMI waveguide.
 10. Method according to claim 9,further comprising the step of adjusting a refractive index, M, of saidphase shifters.
 11. Method according to claim 9, further comprising thestep of adjusting the refractive index, M, of said phase shifters forcontrolling optical path length and a phase front of each of said selfimages.